Jan Schmedes

Imaging the Shallow Crust Using Teleseismic Tomography

Abstract Teleseismic tomography is typically used to image the lithosphere or upper mantle with station spacing on the order of tens to hundreds of kilometers. It is generally presumed that, due to their low‐frequency energy content (0.3–2xa0Hz), body waves generated by teleseismic earthquakes are unable to resolve small‐scale (1–10xa0km) heterogeneity. Here, we take advantage of the 250‐m station spacing of the LaBarge Passive Seismic Experiment to address two questions related to teleseismic tomography. First, we aim to determine whether relative delays in arrival times are present when the station spacing approaches hundreds of meters. Second, we aim to determine whether these measurements can be related to structure in the shallow crust, thus providing information about the subsurface at hydrocarbon exploration scale. Our results indicate that, even at this small station spacing, variability in relative delay‐time measurements shows a clear relation to the shallow structure below the array (top 5xa0km). Inversions for subsurface velocity show that, while the lateral resolution of the inversion is on the order of the station spacing, the steep incidence of the incoming waves yields poor vertical resolution. We anticipate that additional measurements from local and regional earthquakes may be used to enhance vertical resolution.

Earthquakes—a naturally occurring source of low-frequency data

Seismic reflection data contain information at two very different length scales. The long wavelength or lowfrequency information is derived primarily from move-out velocity analysis and provides information on the order of kilometers (i.e., ~1 to 2 km). The short wavelength (highfrequency) information comes from reflection amplitudes and/or changes in those amplitudes with offset (AVO) and provides information at scales of tens of meters (i.e., ~10 to 250m). Between these two length scales is a gap. This missing information (between 1 and 6 Hz) causes a general uncertainty in all seismic-reflection inversions and creates a challenge for full-waveform inversion methods to properly converge without getting stuck in local mathematical minima. This problem is further compounded in regions with complex structure (e.g., basalt flows, sills and dykes, salt bodies, fold and thrust belts, etc.) where it can be difficult to obtain the long-wavelength information reliably from the data, thereby leading to poor imaging. One possible solution for both of these problems is to use local, regional and teleseismic (far-away) earthquakes as a source of low-frequency energy, exploiting the Earth’s naturally occurring seismicity. This paper describes a field experiment that was designed to test this idea.

A Hybrid Joint-inversion Scheme

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